On my front truss, they're all mounted the same way, yet when I pan them in a group the MH2's rotate one way the the MH5's rotate the other way.

Furthermore some but not all of them rotate in opposite directions in the 2D view (i.e. 2 of the MH2's rotate the opposite way from the rest, and they all rotate the opposite way from the MH5's.

The only way I've been able to get them to rotate the same direction was to invert the pan of the MH5's in the patch, and also invert it on the MH5 fixtures themselves.

This strikes me as really weird, as the software, and all the fixtures are from the same company. If it was a mix of brand of fixtures I could see them possibly being backwards, but they should all rotate the same way for a given change on the pan channels.

Any ideas what could be causing this?

I've given up on the 2D View even vaguely approximating reality, as I've got some fixtures that refuse to match the others (the aforementioned MH2s) that even when they are in a straight line in the 2D view and have the exact same settings rotate weirdly (one of them is perpetually about 80 degrees off from the rest, and the other rotates backwards from the rest).

That's exactly the point. The only way I'm able to get them to pan the same way is TO invert the pan. With both types having pan invert off at the fixture one goes one way, the other the other way.

Only way I've been able to correct it and get them to rotate the same way for a given change in pan was to invert them at the fixture.

They also rotate incorrectly in the 2D View, and the really odd bit is some of them go one way, and others the other way. And yes, there are no pan inverts at the patch level. Trust me I've spent more hours trying to figure this out than I can count at this point.